Abstract-Earthworms, Lumbricus terrestris, exposed for 96 h to filter paper saturated with five nominal concentrations of pentachlorophenol (PCP: 5-50 g PCP/cm 2 ), exhibited a 50% lethal concentration (LC50) of 25.0 g PCP/cm 2 (95% confidence interval [CI]: 22.8-27.2) and corresponding whole worm body burden-based 50% lethal dose (LD50) of 877.7 g PCP/g dry mass (95% CI: 828.5-927.0). Linear regression modeling showed that worms increased body concentrations (BC ϭ g PCP/g dry tissue mass) with increasing exposure concentrations (EC) according to BC ϭ 113.5 ϩ 29.5EC (r 2 ϭ 0.94, p ϭ 0.03). Phagocytosis of yeast cells by immunoactive coelomocytes was suppressed only at body concentrations (863.3 g PCP/g dry mass) that approximated the calculated LD50 and overlapped those demonstrating lethality, indicating a sharp transition between sublethal and lethal toxicity. An exposure concentration of 15 g PCP/cm 2 produced significant suppression of phagocytosis of yeast cells by immunoactive coelomocytes. However, the average measured body burden from this group (863.3 g PCP/g dry mass) approximated the estimated LD50, indicating a sharp toxic response slope. Exposure to 10 g PCP/cm 2 with a corresponding body concentration of 501.3 g PCP/g dry mass did not affect phagocytosis. The importance of body burden data is emphasized.
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